Fuel problems?

la islagb

Recruit
Joined
Jun 14, 2009
Messages
2
Any Ideas?
7.4l 454 mercruiser that runs great with a full tank. Once it gets down to about a 1/2 tank I can hardly keep it at idle speed with out it dying out.If accelerating it will stall after 5 or 10 seconds. Then I will restart it and it will do the same thing again. The previous owner had another water seperator filter added by a service center thinking this would solve it. But to no avail, any suggestions would be great.
 

P 0 P E Y E

Chief Petty Officer
Joined
Jun 3, 2009
Messages
441
Re: Fuel problems?

Could the fuel tank vent be clogged?

If it is safe and practical to perform this check, Next time the engine starts to die, loosen up the fuel fill cap and listen for the sound of atmospheric air attempting to equalize with the tank internal pressure or vacuum created by the engine in the tank.

If your vent or tank is not able to allow air in to replace the volume of fuel, the engine will starve for fuel.
 

flargin

Chief Petty Officer
Joined
Aug 13, 2008
Messages
540
Re: Fuel problems?

you may want to compare your fuel pump to your 1/2 tank level.

Your pump may work when there is "positive" pressure from the tank, and may not be able to pull the fuel when it gets to low.

It may just be that is the limit of your fuel pump draw, you may need to re-build/replace the pump.

is this efi or carbed? electric pump?
 

la islagb

Recruit
Joined
Jun 14, 2009
Messages
2
Re: Fuel problems?

Thanks for the info

I talked to some car mechs today and this is what they think. Either the sendind unit in the fuel tank has a hole in it and its drawing air once the gas gets past some sort of tube, or the fuel pump isn't working and the first half of the tank is just being gravity fed. Any thoughts on hooking up a portable tank to the fuel line and keeping it lower than the carb to see if the pump draws fuel? (sorry for the lack of info it's a 1994 with a mechanical pump located on top of the seawater pump)
 

P 0 P E Y E

Chief Petty Officer
Joined
Jun 3, 2009
Messages
441
Re: Fuel problems?

When the engine stalls, see if you can tell if the carburetor is out of gas.

I had a quadrajet once that had a clogged filter at the inlet that acted similar.

Did you check the vent?
 

180shabah

Rear Admiral
Joined
Mar 26, 2005
Messages
4,995
Re: Fuel problems?

Adults only section has a great write up on testing the fuel system, including setting up a test tank and using a clear hose to see if you are sucking air.
 

wakesupreme

Cadet
Joined
Jun 15, 2009
Messages
10
Re: Fuel problems?

I had a similar problem with my boat. It run great until about 3/4 tank then it would die. It would run if I dumped gas directly into the carb but then would die once that gas was used. I checked everything. The fuel pump, filters, etc. It ended up being that the pick up hose in the gas tank had fallen off and was laying in the bottem of the tanks. So it would suck the gas up when it was completely full but since there was no pick up hose once it reached 3/4 it would die. To check your fuel pump, make sure your small gas resevior contanier is full then crank the engine. open that canister up and if it is still full the pump is not working.

hope this helps.
 
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